CNG - Compressed Natural Gas CNG Properties and Safety Questions and Answers

Question 1

In the event of a leak, how will compressed natural gas (CNG) behave in an open environment?

Accepted Answer

It will dissipate rapidly upwards into the atmosphere.

Answer

CNG is lighter than air, with a specific gravity of about 0.6 compared to air's 1.0. This property causes it to rise and disperse quickly when released, reducing the risk of forming a flammable mixture near the ground.

Question 2

A technician is inspecting a CNG vehicle involved in a minor collision. The technician notices a slight hissing sound and detects a gas leak near a cylinder valve. What should be the technician's IMMEDIATE action?

Accepted Answer

Close the manual shut-off valve for that specific cylinder, if accessible.

Answer

In the event of a leak, the first step is to isolate the source of the fuel. Closing the cylinder's manual shut-off valve will stop the flow of high-pressure gas from that specific cylinder, mitigating the hazard. Technicians should never attempt to tighten fittings on a pressurized system.

Question 3

Which of the following represents the correct flammability range for CNG in air?

Accepted Answer

5% to 15%

Answer

CNG has a relatively narrow flammability range. It will only ignite when its concentration in the air is between approximately 5% and 15%. Below 5%, the mixture is too lean to burn, and above 15%, it is too rich.

Question 4

What is the primary function of a Pressure Relief Device (PRD) on a CNG cylinder?

Accepted Answer

To safely vent the gas in the event of high temperature or pressure.

Answer

A PRD is a critical safety feature designed to protect CNG cylinders from rupture. In the event of a fire or other situation causing excessive heat or pressure, the PRD activates to release the gas in a controlled manner.

Question 5

Compared to gasoline, the auto-ignition temperature of CNG is significantly:

Accepted Answer

Higher, making it less likely to ignite from contact with hot surfaces.

Answer

CNG has a high auto-ignition temperature of about 540°C (1200°F), whereas gasoline's is much lower, around 246-280°C (475-536°F). This property makes CNG less likely to ignite accidentally on a hot surface, such as an exhaust manifold.

(CNG) Compressed Natural Gas Practice Test

(CNG) Compressed Natural Gas Practice Test

CNG - Compressed Natural Gas CNG Properties and Safety Questions and Answers